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Neoplasia (Latin, new growth) is an abnormality of cellular differentiation, maturation, and control of growth. Neoplasms are commonly recognized by the formation of masses of abnormal tissue (tumors). The term tumor can be applied to any swelling—and in that context is one of the cardinal signs of inflammation—but today it is used most commonly to denote suspected neoplasm. Neoplasms are benign or malignant depending on several features, chiefly the ability of malignant neoplasms to spread from the site of origin. Benign neoplasms grow but remain localized. Cancer denotes a malignant neoplasm (the term is thought to derive from the way in which the tumor grips the surrounding tissues with claw-like extensions, much like a crab).

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Although a neoplasm may not be difficult to recognize, the process of neoplasia is hard to define. The definition of neoplasm proposed in the early 1950s by Rupert Willis, a British pathologist, is probably the best: “A neoplasm is an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of the surrounding normal tissues and persists in the same excessive manner after cessation of the stimuli that evoked the change.” This definition is analyzed in greater detail in Chapter 18: Neoplasia: II. Mechanisms & Causes of Neoplasia.

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Although all neoplasms possess certain characteristics in common—particularly the capacity for uncontrolled continuous growth (Chapter 19: Neoplasia: III. Biologic & Clinical Effects of Neoplasms)—they vary enormously in their gross and microscopic features. The clinical presentation, behavior, effects, response to therapy (Chapter 19: Neoplasia: III. Biologic & Clinical Effects of Neoplasms), and etiology (Chapter 18: Neoplasia: II. Mechanisms & Causes of Neoplasia) are likewise diverse.

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For these reasons, the classification of neoplasms has major implications for prognosis and therapy. Approaches to the classification of neoplasms are summarized in Table 17-1.

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Table 17–1. Approaches to Classification of Neoplasms.

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